1. Field of the Invention
The present invention generally relates to a high input voltage, high efficiency, fast transient voltage regulator module and, more particularly, to a push-pull forward converter for driving low voltage, high current loads such as computer microprocessors.
2. Description of the Prior Art
Voltage regulator modules (VRMs) are used for a variety of purposes, such as, for example, to provide power to microcomputer processors or other low voltage, high current loads. Today""s high input VRM topologies, with or without isolation, have low efficiency or slow transient response. If the high input-voltage VRMs don""t have an isolation transformer to adjust their duty cycle their efficiency is very low, like the synchronous rectifier buck converter. Although some high input-voltage VRMs have isolation transformers to adjust duty cycle, like forward and flyback VRMs, their efficiency is lower than that of the proposed topology. Other high input-voltage isolated VRM topologies, like flyback forward and asymmetrical half bridge VRMs, have suitable efficiency, but their transient response is very slow.
FIGS. 1, 2, 3, 4, 5 and 6 show prior art high input-voltage VRM topologies. FIG. 1 shows the synchronous rectifier buck converter. For this kind of converter, when the input voltage is high, its duty cycle is very small. As a result, converter efficiency is low and large output filter is needed for asymmetrical transient response. The gate signals of the two MOSFETs, Q1 and Q2, in the synchronous rectifier (SR) buck is complementary. The operation of buck converter is like that of a conventional buck. That is, it converts a high DC voltage from input Vin to a low DC voltage Vo at the load.
FIG. 2 shows a flyback converter. It has a transformer T to adjust duty cycle. In operation, when switch Ql turns on, the energy is stored in the magnetizing inductor by primary winding. At the same time, the secondary diode Q2 is blocked. After Q1 turns off, the energy stored in the magnetizing inductor is transferred to output load through secondary winding and diode Q2. As a result, the energy is transferred from high DC input voltage source Vin to low DC output Vo at the load. However, this flyback converter has a leakage inductance which causes large voltage spike. This requires a high voltage rating device to be used in the converter. On the other hand, its output current is discontinuous. Therefore, larger output filter is needed.
FIG. 3 shows active clamp forward with synchronous rectifier. When switch Q1 turns on, the energy is transferred from high DC input voltage source Vin to low DC output voltage Vo to a load through transformer and switch Q4. After Q1 turns off, Q4 turns off, then Q2 and Q3 turn on. The magnetizing current is discharged by Cd. And the output inductor current is continued by Q3. This converter has higher efficiency than that of the flyback converter. When there is tight transient response requirement, small inductance is required. Under this condition, its efficiency is reduced and its output filter is large.
FIG. 4 shows a push pull converter with center tapped secondary. The gate signals of the two primary switches Q1 and Q2 are symmetrical and out of the phase (180 degrees difference). When switch Q1 turns on, the energy is transferred from high DC input voltage source Vin to low DC output voltage Vo at the load through transformer and switch Q4. The duty cycle of Q1 is limited to be smaller than 50%. After Q1 turns off, Q4 turns off. After 180 degrees from the turn on point of Q1, Q2 and Q3 turn on. The duty cycle of Q2 is smaller than 50%. Like the flyback converter, the transformer leakage inductance induces large voltage spike. Converter efficiency is limited.
FIG. 5 shows an asymmetrical half bridge converter with a current doubler secondary. The gate signals of the two primary switches Q1 and Q2 are complementary. When switch Q1 turns on, the energy is transferred from high DC input voltage source Vin to low DC output voltage Vo to a load through transformer and switch Q4. The duty cycle of Q1 is limited to be smaller than 50%. After Q1 turns off, Q4 turns off, then Q2 and Q3 turn on. The converter efficiency depends on the input voltage. When it transformer turns ratio is low, its efficiency is reduced. And it is a fourth-order system. Its transient is slow.
FIG. 6 shows a flyback forward converter. The gate signals of the two primary switches Q1 and Q2 are complementary. When switch Q1 turns on, the energy is transferred from high DC input voltage source to low DC output voltage load (between A and B) through transformer and switch Q4. After Q1 turns off, Q4 turns off, then Q2 and Q3 turn on. Cd through Q2 discharges the magnetizing current. Like the asymmetrical half bridge, it is a fourth-order system. Large output filter is needed. Control design is complicated.
It is therefore an object of the present invention to provide a VRM push-pull forward topology having high efficiency and fast transient response.
According to the invention a VRM has a primary side wherein the two switches and two primary transformer windings are alternately connected in a loop. A capacitor is connected between any of two interleaved terminations. The remaining two terminations are connected to input and ground respectively. The two primary transformer windings have the same number of turns. A number of secondary sides may be used such as, for example, a half wave rectifier or a center tapped secondary.
The novel high input-voltage push-pull forward VRM has high efficiency and fast transient response. Compared with conventional high input-voltage VRMs, its filter capacitance and inductance can be reduced 2-4 times. Its magnetic components can be easily integrated. As a result, very high power density can be achieved. Also, compared with conventional VRMs, its efficiency is high. The device die size needed to achieve the required efficiency is reduced. And control is simple. Therefore, this topology is very cost effective.